To find similar items to a certain item, you’ve got to first define what it means for 2 items to be similar and this depends on the problem you’re trying to solve:

on a blog, you may want to suggest similar articles that share the same tags, or that have been viewed by the same people viewing the item you want to compare with
Amazon has this section called “customers that bought this item also bought”, which is self-explanatory
a service like IMDB, based on your ratings, could find users similar to you, users that liked or hated approximately the same movies you did, thus giving you suggestions on movies you’d like to watch in the future
In each case you need a way to classify these items you’re comparing, whether it is tags, or items purchased, or movies reviewed. We’ll be using tags, as it is simpler, but the formula holds for more complicated instances.